Phase transitions are ubiquitous in nature. The liquid-gas transition or the transition from a magnetized to a non-magnetized state in ferromagnetic materials are textbook examples [1]}, [2]}, [3]}. Critical phenomena of continuous phase transitions have been a major research topic for more than 50 years. It is now well understood that, at equilibrium, the thermal fluctuations play a dominant role: the amplitude of the order parameter, say \(A\) , depends on the distance from the critical point, say \(\mu \) , as a power-law \(A\propto \mu ^\beta \)  where the value of the exponent \(\beta \)  differs from the mean-field prediction obtained when thermal fluctuations are neglected. These results are verified in experiments and are understood theoretically for instance through renormalization methods. In contrast, the behavior of critical phenomena in non equilibrium systems remains  less well understood. In liquid crystals, a transition between two topologically different nematic phases was shown to belong to the class of directed percolation [4]}. The transition from the laminar state to turbulence in extended shear flows [5]}, [6]}, [7]} is an out of equilibrium phase transition that also belongs to the directed percolation universality class.
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